Workers can be running on any computer, as long as they have access to the database and their clock is in sync. Keep in mind that each worker will check the database at least every 5 seconds.

You can also invoke rake jobs:work which will start working off jobs. You can cancel the rake task with CTRL-C.

Custom Jobs

Jobs are simple ruby objects with a method called perform. Any object which responds to perform can be stuffed into the jobs table. Job objects are serialized to yaml so that they can later be resurrected by the job runner.

Gory Details

The library evolves around a delayed_jobs table which looks as follows:

create_table :delayed_jobs, :force => true do |table|
table.integer :priority, :default => 0 # Allows some jobs to jump to the front of the queue
table.integer :attempts, :default => 0 # Provides for retries, but still fail eventually.
table.text :handler # YAML-encoded string of the object that will do work
table.text :last_error # reason for last failure (See Note below)
table.datetime :run_at # When to run. Could be Time.zone.now for immediately, or sometime in the future.
table.datetime :locked_at # Set when a client is working on this object
table.datetime :failed_at # Set when all retries have failed (actually, by default, the record is deleted instead)
table.string :locked_by # Who is working on this object (if locked)
table.timestamps
end

On failure, the job is scheduled again in 5 seconds + N ** 4, where N is the number of retries.

The default Worker.max_attempts is 25. After this, the job either deleted (default), or left in the database with “failed_at” set.
With the default of 25 attempts, the last retry will be 20 days later, with the last interval being almost 100 hours.

The default Worker.max_run_time is 4.hours. If your job takes longer than that, another computer could pick it up. It’s up to you to
make sure your job doesn’t exceed this time. You should set this to the longest time you think the job could take.

By default, it will delete failed jobs (and it always deletes successful jobs). If you want to keep failed jobs, set
Delayed::Worker.destroy_failed_jobs = false. The failed jobs will be marked with non-null failed_at.

Commit the changes without making changes to the Rakefile, VERSION, or any other files that aren’t related to your enhancement or fix

Send a pull request.

Changes

1.7.0: Added failed_at column which can optionally be set after a certain amount of failed job attempts. By default failed job attempts are destroyed after about a month.

1.6.0: Renamed locked_until to locked_at. We now store when we start a given job instead of how long it will be locked by the worker. This allows us to get a reading on how long a job took to execute.

1.5.0: Job runners can now be run in parallel. Two new database columns are needed: locked_until and locked_by. This allows us to use pessimistic locking instead of relying on row level locks. This enables us to run as many worker processes as we need to speed up queue processing.